The invention relates to a reception method used in a radio system for synchronization to a received signal, the radio system comprising at least one receiver receiving modulated and partly previously known signal consisting of symbols and the signal,containing time and frequency deviation.
The invention further relates to a receiver used in a radio system in which synchronization is made to the received signal, the radio system comprising at least one receiver receiving modulated and partly previously known signal consisting of symbols and the signal containing time and frequency deviation.
In radio systems a user data signal usually modulates a high frequency signal transmitted through the radio path to a receiver. The receiver can be located very far from a transmitter depending on the radio system. For example, in satellite radio systems a base station functioning as a transceiver can be located at a height of 38000 km above the equator. On the radio path between a transmitter and a receiver the signal is always subjected to interference that impair the signal properties. In satellite radio systems the base station typically moves in relation to the earth in such a way that seen from the ground the base station is stationary. The base stations can also be on relatively low orbits, the satellites then moving in relation to the earth faster than the satellites of said satellite system. The movement of satellites causes big frequency and time delay variations.
It is typical for a radio system environment that a signal travelling between a user and a base station does not travel straight but depending on the environment the signal propagates on paths of various lengths from the transmitter to the receiver. Such a multipath propagation occurs even though there is a direct line of sight between the base station and the mobile station. The multipath propagation results from the signal being reflected and scattered in the peripheral interfaces it encounters. Signals propagating on different paths have a propagation delay of a different length, the signals thus arriving at the receiver in different phases.
On the radio path various forms of interference modify the modulated signal, thus significantly complicating the separation or demodulation, of the data signal in the receiver. As the propagation time of the signal transmitted by the transmitter is limited, the signal is subjected to delay. The longer the distance between the transmitter and the receiver, the more delay is generated. Furthermore, the movement of the base station or the subscriber terminal, for example a mobile phone, produces Doppler frequencies. The Doppler frequencies change the signal frequency received by the receiver. The signal frequency change causes problems for the synchronization of the receiver to the received signal.
The above described delay and frequency problems have been avoided using various algorithm methods such as methods based on correlation. However, some restrictions have been placed on the methods based on correlation. For example, a very small frequency error of the signal received by the receiver has been allowed in order to enable synchronization by using the method. Sufficient time synchronization has not been achieved with prior art solutions.
A prior art method is based on indicating the signal burst power transmitted by the transmitter. In this method a sliding window is used for estimating signal power and noise power. Signal power is compared with noise power, and the disadvantageous signal properties can then be estimated. However, it is only possible to use the method for estimating a bursty signal. In addition, the method has not been applicable to radio systems with a small signal-to-noise ratio. Consequently prior art methods have not been well applicable to e.g. satellite radio systems.
An object of the present invention is to implement a method in which the receiver can synchronize to a signal it has received, the signal containing a substantial amount of changes caused by delay and frequency.
This is achieved with the method set forth in the preamble characterized in that the received signal is multiplied by the known part of the signal, whereby a product is obtained, the product is correlated, whereby a ratio is obtained, the ratio is then compared with a pre-set correlation threshold value on the basis of which comparison a decision is made on synchronization to the received signal.
The receiver of the invention is characterized in that the receiver comprises multiplication means for multiplying the received signal by the known part of the received signal in order to obtain the product, transform means for correlating the product, whereby a ratio is obtained, and comparison means for comparing the ratio with a pre-set correlation threshold value on the basis of which comparison a decision is made on synchronization to the received signal.
Great advantages are achieved with the method of the invention. In this method a part of the received signal is known. The received signal is multiplied by the known part of the received signal such as training sequences. Signal multiplication removes signal modulation when the received signal corresponds with the known part. The modulation is removed with an accuracy of one symbol. A product preferably correlated using FFT-transform is obtained by multiplying the signal. The correlation is performed substantially simultaneously at time and frequency level. Correlation allows to decide on synchronization to the received signal and to calculate even a big frequency deviation in the received signal. In this method synchronization is made to the signal if the ratio obtained on the basis of the correlation exceeds the pre-set correlation threshold value. The method is further applicable to signals with a small signal-to-noise ratio.